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Relapse during rituximab (RTX)-based maintenance therapy remains a clinical problem in ANCA-associated vasculitis (AAV). Mycophenolate mofetil (MMF) has been investigated as an alternative; however, evidence regarding its use after relapse during RTX-based maintenance is limited. We aimed to retrospectively describe clinical outcomes in patients with AAV who relapsed during RTX-based maintenance according to whether MMF was included in the initial post-relapse treatment strategy. We retrospectively analyzed 17 patients with AAV who experienced their first relapse during RTX-based maintenance after remission induction. Baseline was defined as the time of the first relapse. Post-relapse treatment included glucocorticoid escalation and/or RTX re-administration; patients were categorized according to whether MMF was additionally included in the initial treatment strategy (MMF-added group, n = 9; MMF-not-added group, n = 8). All outcomes were evaluated descriptively. In the MMF-added group, MMF was initiated at a median of 0.4 months after the first relapse (interquartile range [IQR], 0.3-0.4 months; range, 0.3-0.5 months). The median age was 76 years (IQR, 73-81 years); 16 patients (94.1%) were MPO-ANCA-positive. During a median follow-up of 28 months (IQR, 21-33 months), all patients achieved remission after post-relapse treatment intensification. No second relapse occurred in the MMF-added group, whereas three patients in the MMF-not-added group experienced a second relapse. Glucocorticoids were discontinued in all nine patients in the MMF-added group, whereas in the MMF-not-added group, all eight patients continued glucocorticoids while they were managed without MMF. Severe infections requiring hospitalization occurred in none of the patients in the MMF-added group and in three patients in the MMF-not-added group. Because treatment allocation was non-randomized and post-relapse treatment strategies differed with respect to RTX scheduling, concomitant therapies, and physician-directed glucocorticoid tapering, the independent effect of MMF could not be determined. Therefore, these descriptive and hypothesis-generating findings warrant confirmation in future prospective studies.